1. Field of the Invention
The invention relates to laser beam generating apparatus and more particularly to light beam sources having minimal diameters for optical recording, laser printing, fiber optic communications, microetching, and the like.
2. Description of the Prior Art
The optical spot made by a lens with a circular aperture is well known and described in the literature such as:
Born and Wolf, "Principles of Optics"; Permagon Press, New York 1964, pg 395.
Hecht and Zajac, "Optics"; Addison Wesley, Menlo Park, Calif. pg 350.
A prior patent to Adachi is also related; U.S. Pat. No. 5,321,717 provides an optical element structure for reducing beam spot size. However, the search for even smaller light beam spot size is fruitful in data recording/transmission by compact disk and other media since the resolution, and thus the total amount of data that can be stored in a given area, can be greatly increased in relation to the decrease in spot size.
The optical spot made by a lens with a circular aperture is: ##EQU1##
The intensity distribution I(p)={U(p)}
J1 is a Bessel function of the 1st kind and the 1st zero is 3.83. k=2.pi./(wavelength), a is a radius of aperture and can be replaced by NA (numerical aperture (a/fl) where fl is focal length PA1 w is a distance from the center of the image.
Then kaw=3.83 ##EQU2## Where wavelength=0.65 .mu.m, Diode laser visible red, NA=0.65 possible largest NA with long working distance. The spot size is 2a=1.2 .mu.m. This will be smaller for shorter wave length and larger NA. However, both requirements are faced to the limit. Even with future improvement, spot size is limited to 1 .mu.m.
If an ultraviolet laser is to be used, the instrument will be heavy and bulky, and the lens no longer able to use optical glass of high refractive index: and the NA will be smaller by 0.4.
The theoretical limit of resolution is the laser wavelength. If the wavelength, in the case of a diode laser, is 630 to 780 nanometers: for example 633 nanometers. proportionately, using a shorter wavelength beam, such as an Argon laser beam which has a 460 nanometer wavelength, then a spot less than 400 nanometers is obtained, which is much smaller than what is currently being used in Integrated Circuit (IC) processing by photoetching.
Scanners for photoetching functions advantageously are moving to the use of diode lasers instead of Argon lasers now in use. The Argon laser is prohibitively expensive since external modulators must be used resulting in very high equipment costs. By using diode lasers, modulation capability is intrinsic in the design, and the longer wavelength, contrasted with the Argon laser, can be compensated by use of the novel Bessel Transform circular grating optical filter of the invention, to obtain the high resolution beam spot equivalent to use of the Argon laser.
Use of the novel filter in combination with the diode laser is an inexpensive method for obtaining the high resolution 1600 lines per inch required for Integrated Circuit scanners and other high resolution applications over 600 lines per inch.
Accordingly, it is a primary object of the invention to provide a means for reducing laser beam spot size to increase the resolution of the beam particularly for applications requiring resolution of upwards of 600 lines per inch. Such applications exist in laser printing, and in manufacturing of Integrated Circuits, by high resolution photo etching wherein using shorter wavelength lasers such as the Argon laser to obtain higher resolution may be cost prohibitive.
It is another object of the invention to provide a general solution to beam spot size reduction that is easily accomplished by a simple filter that can be added to existing optics systems to provide nominally 30 percent reduction in spot size at any laser wavelength. For example the Argon Laser has half the wavelength of conventional diode lasers, and a beam spot size of one micron. If the novel filter is used with an Argon laser, an even smaller spot can be obtained. The micron beam diameter can be made smaller by thirty percent or 0.7 micron.
Yet another object of the invention is to provide a resolution enhancing filter that can be accurately and inexpensively produced by photo-resist methods. The geometry of the novel Bessel order circular grating filter is easily accomplished by making calculations to determine the Bessel order ring diameters, and etching a substrate using normal photoresist techniques to make one master. The master can be large; i.e., the size of the numerical aperture in the associated system optics since the novel filter is positioned before the focus or imaging optics. The master can be duplicated for any laser system application requiring resolution of 1600 lines per inch such as IC photoetching scanners, or lesser requiremnents such as 300 to 600 lines per inch in laser printers.